1. Field of the Invention
The present invention relates to an estimated vehicle speed computing device which estimates the vehicle speed from the rotational velocity of the wheels and, more particularly, to an estimated vehicle speed computing device particularly suitable for use in an anti skid brake control system.
2. Description of the Prior Art
An anti skid brake control system (ABS) compares the rotational velocity of a wheel with the vehicle speed to detect symptoms of wheel locking. When these symptoms are detected, the ABS overrides the driver's operation to apply braking, automatically reduces the brake pressure and weaken the braking effect, thus relaxing the locking tendency of the wheels. To detect these wheel locking symptoms, it is necessary to know the vehicle speed and the most common method used is to estimate the vehicle speed from the wheel velocity.
As shown in FIG. 5, however, because it is not possible for the vehicle speed to rise as quickly as the rotational velocity of the drive wheel during sudden acceleration, the drive wheel spins and the vehicle gradually accelerates until a stable, constant relationship between the rotational velocity of the drive wheel and the vehicle speed is established. If the vehicle speed is estimated based upon the rotational velocity of the drive wheel during rapid acceleration, there will be a major difference between the estimated and actual vehicle speeds, and the estimated vehicle speed will have no practical meaning. Methods were therefore proposed, in U.S. Pat. No. 4,957,329 issued on Sep. 18, 1990, to prevent this large difference by prohibiting the output of the estimated vehicle speed for a constant period Tp after rapid vehicle acceleration was detected, and to estimate the vehicle speed after the passage of this predetermined wait period.
However, because the vehicle speed during rapid acceleration and the rotational velocity of the drive wheel change as shogun in FIG. 5, if the output of the estimated vehicle speed is simply prohibited for a constant period Tp after rapid vehicle acceleration is detected, if the convergence between drive wheel and vehicle speed is delayed, i.e., if spinning of the drive wheel does not end within the period of Tp but ends at time .alpha. passed after the period Tp, the estimated vehicle speed will be greater than the actual speed, causing locking symptoms to be erroneously detected and an brake pressure to be unnecessarily reduced.